In recent years lighting devices that use light emitting diodes (hereinafter referred to as “LED(s)”) have been under development, and some are being put into practical use.
One example of a lighting device that uses LEDs (hereinafter referred to as an “LED lighting device”) is one in which LED bare chips are mounted on a substrate (this arrangement is called an “LED module”), an the LED bare chips are made to emit light according to power from a power supply source. A plurality of LED bare chips are generally mounted on the substrate because sufficient light to produce a lighting device is not provided by only one LED bare chip. The LED bare chips are mounted densely in order to produce a more compact lighting device.
In an LED lighting device with such a structure, the LED bare chips exhibit premature deterioration due to the heat generated by the LED bare chips themselves during operation. For this reason, consideration is being given to using metal base substrates due to their high thermal conductivity compared to resin substrates. A metal base substrate has a layered structure that includes a metal layer and an insulative layer (resin), and has a thermal conductivity of approximately 1 W/mK to 10 W/mK.
Furthermore, in order to stabilize the luminous intensity of the LED bare chips during operation in an LED lighting device, power from a power supply source is controlled so as to have a constant current (see Japanese Patent Application Publication No. 2001-215913).
When an LED module has reached the end of its life expectancy, it is necessary to replace the LED module. However, a problem arises that the specifications of the replacement LED module differ from those of the original LED module.
Specifically, LEDs have a significantly longer life expectancy than conventional incandescent lamps, and with rapid progress in the development of LEDs, it is unlikely that the specifications (for example the Vf of the LED bare chips) of LED modules at the time of replacement will be the same as the specifications when the lighting device was designed.
In terms of a device that uses the circuit described in the aforementioned patent document, the circuit structure of the device is such that the LED module and the circuit are separate, and the circuit is composed of a converter circuit and a constant current circuit.
With this circuit, when a number of LED modules are provided in parallel, there is only one converter circuit feedback signal. Even if the number of LED modules is increased, there is still only one main LED module used as a reference.
In other words, the control depends strongly on the LED module connected to extract the feedback signal, and control of other LED modules becomes dependant on the main LED module. This is not ideal for the LED modules. For this reason, when replacing the LED module in this device, it is preferable to use an LED module that has the same properties (specifications) as the original LED modules.
If a unit made up of a most current LED module is used to replace the main LED module, the capability of the dependant LED modules will be reduced. In the same way, if a dependant LED module is replaced, the capability of the replacement dependant module will suffer.
In this way, according to the aforementioned patent document, it is difficult to obtain maximum performance from each LED module because it is not possible to compensate for differences in LED performance of the LED modules.
For this reason, in order to maintain LED module performance in such devices, it is necessary to either recommence manufacturing of LED modules with the specifications at the time of design, or to keep a stock of such LED modules. Furthermore, an LED module cannot be replaced with the most current LED module that is superior in aspects such as Vf of the LED bare chip.